Neurons make synaptic connections with a precise specificity in order to assemble neural circuits. Although molecules that control initial axonal trajectories are relatively well understood, it is largely unknown how precise synaptic connections are formed within the target area. The goal of this proposal is to understand the molecular basis of synapse formation and synaptic specificity in the developing mouse spinal cord. The spinal reflex circuit in the spinal cord is an excellent model system to study synapse formation and synaptic specificity because of its relative simplicity and availability of abundant knowledge based on previous anatomical and electrophysiological studies. Cell bodies in the ventral gray matter are grouped into motor neuron pools, which project axons to specific muscles. There are approximately fifty such motor neuron pools at the levels of the limbs in the mouse spinal cord. Proprioceptive sensory neurons, which innervate these muscles with proprioceptive fibers, have their cell bodies in the dorsal root ganglia (DRG), and project axons into the spinal cord, which terminate and make synapses with the appropriate motor neuron pools. Our preliminary data strongly suggest that the semaphorin (sema) family of signaling ligands, and their receptors, the plexins, control synapse formation and synaptic specificity of the sensory-motor connections. First, of all semas and plexins, expression of only plexinA1, plexinD1, and sema6B is highly enriched in proprioceptive sensory neurons. Second, sema6D and its receptor plexinA1 are expressed by motor and proprioceptive sensory neurons when synaptogenesis is occurring. Third, plexinD1 is expressed by subsets of proprioceptive sensory neurons, while its ligand sema3E is expressed by subsets of motor neurons. We hypothesize that sema-plexin combinations contro l synapse formation and synaptic specificity in the developing spinal cord. The first aim will examine whether sema6D-plexinA1 signaling regulates synapse formation of sensory-motor connections. The second and third aims will examine if sema3E- plexinD1 and plexinA4-sema6B signaling control synaptic specificity of sensory-motor connections. We will address these issues by using a combination of anatomical, electrophysiological, behavioral, and in vitro analyses together with mouse genetics. PUBLIC HEALTH RELEVANCE: The spinal cord represents the region of the central nervous system (CNS) concerned with motor behavior. Malfunction of the motor neurons in the spinal cord occurs in a number of neurological disorders such as amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA). These motor neuron diseases are a group of progressive disorders that destroy cells that control essential muscle activity such as speaking, walking, breathing, and swallowing. Messages from nerve cells in the brain are transmitted to nerve cells in the spinal cord as well as the brain stem and from them to particular muscles. The activity of motor neurons is also controlled by proprioceptive sensory neurons of the dorsal root ganglia (DRG) in the peripheral regions. Therefore, it is important to understand how motor neurons are controlled peripherally (by proprioceptive sensory neurons) as well as centrally (by cortical neurons in the brain) in order to develop the therapy for motor neurons diseases. Furthermore, spinal cord injury causes disfunction of motor neurons, and since many other human neurological disorders are caused by defects in neural circuit formation, understanding the mechanism of neural circuit formation will uncover an underlying cause of the diseases. In this proposal, we will test our hypothesis that semaphorin-plexin signaling controls correct sensory-motor connectivity in the mouse spinal cord. We believe that better understanding of the molecular mechanisms underlying the formation of sensory-motor connections is likely to contribute to advancing diagnosis, therapy, and prevention of neurological disorders.